1. Field of the Invention
This invention relates to a radiation image read-out apparatus, and more particularly to a radiation image read-out apparatus for reading out a radiation image stored on a stimulable phosphor sheet by the use of a line sensor.
2. Description of the Related Art
When certain kinds of phosphor are exposed to a radiation, they store a part of energy of the radiation. Then when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light or a laser beam, light is emitted from the phosphor in proportion to the stored energy of the radiation. A phosphor exhibiting such properties is generally referred to as xe2x80x9ca stimulable phosphorxe2x80x9d. In this specification, the light emitted from the stimulable phosphor upon stimulation thereof will be referred to as xe2x80x9cstimulated emissionxe2x80x9d. There has been known a radiation image read-out apparatus in which a stimulating light beam such as a laser beam is caused to scan a stimulable phosphor sheet (a sheet provided with a layer of the stimulable phosphor) which has been exposed to a radiation passing through an object such as a human body to have a radiation image of the object stored on the stimulable phosphor sheet, the stimulated emission emitted from the stimulable phosphor sheet pixel by pixel is photoelectrically detected, thereby obtaining an image signal (a radiation image signal), and then the stimulable phosphor sheet is exposed to erasing light after the image signal is obtained from the stimulable phosphor sheet so that the residual energy of the radiation is fully released from the stimulable phosphor sheet.
The radiation image signal thus obtained is subjected to image processing such as gradation processing and/or frequency processing and a radiation image of the object is reproduced as a visible image for diagnosis on the basis of the processed radiation image signal on a recording medium such as a photographic film or a display such as a fine CRT. When the stimulable phosphor sheet is exposed to erasing light, the residual energy of the radiation is fully released from the stimulable phosphor sheet and the stimulable phosphor sheet comes to be able to store a radiation image again, whereby the stimulable phosphor sheet can be repeatedly used.
In the radiation image read-out apparatus, it has been proposed to use a line light source which projects a line beam onto the stimulable phosphor sheet as a stimulating light source and to use a line sensor having an array of photoelectric convertor elements as a means for photoelectrically reading out the stimulated emission. The line beam is moved relative to the stimulable phosphor sheet and the line sensor in the direction perpendicular to the longitudinal direction of the line beam by a scanning means. By the use of a line beam and a line sensor, the reading time is shortened, the overall size of the apparatus can be reduced and the cost can be reduced. See, for instance, Japanese Unexamined Patent Publication Nos. 60(1985)-111568, 60(1985)-236354, and 1(1989)-101540.
In the conventional radiation image read-out apparatus using such a line sensor, there has been proposed a system in which is employed a refractive index profile type lens array such as a SELFOC(copyright) lens array, a rod lens array or the like, which is formed by an imaging system where the object plane and the image plane are in one to one correspondence, in order to increase convergence of the stimulated emission on the line sensor. The refractive index profile type lens array comprises a plurality of refractive index profile type lenses which are arranged according to the arrangement of the photoelectric convertor elements in the line sensor.
For example, when the photoelectric convertor elements are arranged as shown in FIG. 2 in the line sensor, the refractive index profile type lenses in the refractive index profile type lens array are arranged as shown in FIG. 3.
Use of such a refractive index profile type lens array has involved the following problem. That is, in the refractive index profile type lens array, there are formed non-aperture portions, where no lens exists, between lenses. The convergence of the stimulated emission is naturally lower in areas corresponding to the non-aperture portions, which results in a periodic pattern like stripes which extend in the direction perpendicular to the longitudinal direction of the line sensor and appear in a reproduced image at the pitches of the non-aperture portions.
In view of the foregoing observations and description, the primary object of the present invention is to provide a radiation image read-out apparatus in which the periodic pattern like stripes due to the non-aperture portions in the refractive index profile type lens array is suppressed.
In accordance with the present invention, there is provided a radiation image read-out apparatus comprising a line stimulating light beam source which projects a line stimulating light beam onto a stimulable phosphor sheet storing thereon radiation image information, a line sensor which comprises a plurality of photoelectric convertor elements arranged in the longitudinal direction of the line irradiated portion to receive stimulated emission emitted from the irradiated portion of the stimulable phosphor sheet or the back side of the sheet opposed to the line irradiated portion and convert the amount of stimulated emission to an electric signal, a light collector means which is disposed between the stimulable phosphor sheet and the line sensor and includes a refractive index profile type lens array which converges the stimulated emission onto the respective photoelectric convertor elements of the line sensor, a scanning means which moves the stimulating light beam source and the line sensor relatively to the stimulable phosphor sheet in a direction different form said longitudinal direction, and an image signal read-out means which reads out the output of the line sensor in sequence in the respective positions in which the stimulating light beam and the line sensor are moved by the scanning means and reads out an image signal representing the radiation image information stored in the stimulable phosphor sheet, wherein the improvement comprises that
the photoelectric convertor elements of the line sensor and the refractive index profile type lenses of the refractive index profile type lens array are arranged at pitches such that the frequency band of the periodic pattern generated due to the pitches of the refractive index profile type lenses in said refractive index profile type lens array are higher than the frequency band of a radiation image information reproduced on the basis of the image signal.
The frequency band of a radiation image information reproduced on the basis of the image signal varies by the kind of the radiation image. In the case of a normal radiation image, the frequency band is 3 to 5 cycle/mm and in the case of a radiation image where a high sharpness is required, the frequency band is 5 to 10 cycle/mm. In this invention, the pitches of the photoelectric convertor elements of the line sensor and the refractive index profile type lenses of the refractive index profile type lens array are set to be higher than the frequency band.
As the line stimulating light beam source, a fluorescent lamp, a cold cathode tube, an LED array and the like can be employed. The line stimulating light beam source itself need not be linear so long as the stimulating light is projected onto the stimulable phosphor sheet in the form of a line beam. That is, the line stimulating light beam source may be provided with an optical system which shapes light emitted from the light source into a line beam. Further, a broad area laser may be employed as the linear stimulating light beam source. The stimulating light beam may be continuously emitted from the light beam source or may be emitted therefrom in a pulse-like fashion. From the viewpoint of reduction in noise, preferably the line stimulating light beam is in the form of high output pulsed light.
It is preferred that the length of the line stimulating light beam on the stimulable phosphor sheet (or the irradiated area of the stimulable phosphor sheet irradiated by the line stimulating light beam) be equivalent to or larger than the length of the side of the stimulable phosphor sheet. In this case, the line stimulating light beam may be projected obliquely to the side of the stimulable phosphor sheet.
In order to increase the degree of convergence of the stimulating light beam on the surface of the stimulable phosphor sheet, an optical system comprising a cylindrical lens, a slit, a SELFOC lens array, an optical fiber bundle, or a combination of these elements may be provided between the stimulating light beam source and the stimulable phosphor sheet. When an optimal second stimulating wavelength of the stimulable phosphor sheet is 600 nm or so, it is preferred that the phosphor be activated with Eu3+ (luminescent center) and the stimulable phosphor layer be supported on a support sheet of glass or high polymer.
The width of the stimulating light beam on the surface of the stimulable phosphor sheet is preferably 10 to 4000 xcexcm.
As the line sensor, an amorphous silicon sensor, a CCD sensor, a CCD with back illuminator, a MOS image sensor or the like may be used.
The refractive index profile type lens array of the light collector means generally comprises a SELFOC(copyright) lens array or rod lens array which is formed by an imaging system where the object plane and the image plane are in one to one correspondence in order to collect the stimulated emission emitted from respective parts of the stimulable phosphor sheet, and is generally formed of glass or high polymer material.
It is further preferred the light collector means is provided, in addition to the refractive index profile type lens array, with at least one of a cylindrical lens, a slit, an optical fiber bundle and the like.
It is further preferred that a stimulating light cut filter (a sharp cut filter, a band pass filter and such) which does not transmit the stimulating light but transmits the stimulated emission be provided between the stimulable phosphor sheet and the line sensor to prevent the stimulating light from entering the line sensor.
The light receiving face of each of the photoelectric convertor elements of the line sensor is set to be smaller than the width of the stimulated emission as seen on the light receiving face of the line sensor for the width of the irradiated area described above, and a plurality of photoelectric convertor elements are arranged in the longitudinal direction of the stimulated emission so that the overall line sensor is equal to or larger than the stimulated emission in length.
The line sensor may be provided with a plurality of photoelectric convertor elements also in the direction perpendicular to the longitudinal direction of the line stimulating light beam projected onto the stimulable phosphor sheet. In this case, the photoelectric convertor elements need not be arranged in a straight line in each of the longitudinal and transverse directions of the line sensor but may be arranged in other patterns. For example, the photoelectric convertor elements may be arranged zigzag in the transverse direction of the stimulated emission and arranged in a straight line in the longitudinal direction, and may be arranged zigzag in the longitudinal direction of the stimulated emission and arranged in a straight line in the transverse direction. Further, the photoelectric convertor elements may be arranged zigzag in both the longitudinal direction and the transverse direction.
When the number of photoelectric convertor elements is large to such an extent that influence of transfer rate is recognizable, shortening of charge accumulating time due to increase in charge transfer time may be avoided by once storing the charge accumulated in each photoelectric convertor element in a memory, and reading out the charge from the memory during a next charge accumulating cycle.
It is preferred that the line sensor includes not less than 1000 photoelectric convertor elements in the longitudinal direction thereof, and that the light receiving face of the line sensor be not shorter than the corresponding side of the stimulable phosphor sheet.
The direction in which the scanning means moves the line stimulating light beam source and the line sensor relatively to the stimulable phosphor sheet is preferably a direction substantially perpendicular to the longitudinal direction of the line stimulating light beam source and the line sensor but maybe any direction so long as substantially the entire surface of the stimulable phosphor sheet can be uniformly exposed to the stimulating light beam. For example, when the line stimulating light beam source and the line sensor are longer than the side of the stimulable phosphor sheet, the scanning means may move obliquely or zigzag the line stimulating beam source and the line sensor relatively to the stimulable phosphor sheet.
The line sensor may receive stimulated emission from the same side of the stimulable phosphor sheet as the side onto which the stimulating light beam is projected or from the back side of the stimulable phosphor sheet opposite to the side onto which the stimulating light beam is projected. In the latter case, the support sheet on which the stimulable phosphor layer is supported should be permeable to the stimulated emission.
When the photoelectric convertor elements of the line sensor and the refractive index profile type lenses of the refractive index profile type lens array are arranged at pitches such that the frequency band of the periodic pattern generated due to the pitches of the refractive index profile type lenses in said refractive index profile type lens array are higher than the frequency band of a radiation image information reproduced on the basis of the image signal as in the present invention, the periodic pattern is included in the image signal as aliasing noise. Accordingly, it is preferred that the radiation image read-out apparatus of this invention be further provided with a frequency component removal means which removes a frequency component corresponding to the frequency band of the periodic pattern from the image signal.
The frequency component removal means may comprise an optical low-pass filter disposed upstream of the line sensor, or an electric low-pass filter or digital filter which removes aliasing noise from the image signal.
It is further preferred that the pitch S of the refractive index profile type lenses be not larger than double the pitch L of the photoelectric convertor elements of the line sensor (Sxe2x89xa62L).
Further, it is preferred that the pitch L of the photoelectric convertor elements of the line sensor be in the range of 25 xcexcm to 250 xcexcm. Though it is preferred that the pitch S of the refractive index profile type lenses be as small as possible, the pitch S of the refractive index profile type lenses is preferably in the range of 10 xcexcm to 500 xcexcm from the viewpoint of easiness of manufacture.
In accordance with the present invention, since the photoelectric convertor elements of the line sensor and the refractive index profile type lenses of the refractive index profile type lens array are arranged at pitches such that the frequency band of the periodic pattern generated due to the pitches of the refractive index profile type lenses in said refractive index profile type lens array are higher than the frequency band of a radiation image information reproduced on the basis of the image signal, unevenness due to the non-aperture portions of the refractive index profile type lens array is not included in the image signal, whereby a sharp image free from stripe-like unevenness due to the non-aperture portions of the refractive index profile type lens array can be obtained.
Further, by removing a frequency component corresponding to the frequency band of the periodic pattern from the image signal, a sharper image free from aliasing noise can be obtained.